Not sure which toy car I'll be using. I just open one and there's one motor for the back wheels but that won't do as I'm changing plans. I'm thinking of a two wheeled platform that can turn around on the spot, running one of the wheels forward and the other backward.

Yes, there is absolutely a trade-off between trying to do everything yourself on the lowest possible budget, versus ease and predictability of getting to the point where you have a robot that you can program to do robot-ey things!If you enjoy the make-it-yourself part, that's certainly a fine path to go (and one I've gone, too.)

However, if you are not confident in being able to build the platform inquestion, and you end up paying e40 for parts and shipping in the end, and then have to throw it away, and pay another e75 for the 3pi and programmer to get something working, that's more expensive than just paying the e75 to begin with. Only you can make the right trade-off here!